The present invention relates to an exhaust passage structure of a multi-cylinder engine.
In an engine equipped with an exhaust turbocharger, the exhaust gas from plural cylinders flows down in an exhaust manifold toward a collective portion. The exhaust turbocharger is arranged downstream of the collective portion. It is preferable that the exhaust gas from exhaust ports of the engine, i.e., the blow down gas, be supplied to the exhaust turbocharger without weakening its flowing-down power in order to make the exhaust turbocharger operate efficiently.
Japanese Patent Laid-Open Publication No. 2007-231791 proposes the structure in which the exhaust turbocharger is arranged downstream of the collective portion where the independent passages of the exhaust manifold collect, and the nozzle to direct the exhaust gas flow toward the center of the collective portion is arranged at the collective portion, so that the exhaust gas can be supplied to the exhaust turbocharger with the properly strong flowing-down power.
In general, the exhaust passage upstream of the exhaust turbocharger branches into plural independent passages which are coupled to the cylinders which are different from each other. Herein, the gas flowing downward in a specified independent passage flows toward another independent passage and expands, so that the flowing-down power of the exhaust gas flowing to the exhaust turbocharger decreases. Consequently, it may be difficult to make the exhaust turbocharger operate efficiently. According to the structure of the above-described publication, while the exhaust gas flowing down into the collective portion is concentrated toward the exhaust turbocharger, it may be difficult that the gas from a specified independent passage is prevented from flowing into another independent passage.